We use observations of the absorption properties of black carbon and
non-black carbon impurities in near-surface snow collected near the research
stations at South Pole and Dome C, Antarctica, and Summit, Greenland, combined
with a snowpack actinic flux parameterization to estimate the vertical
profile and e-folding depth of ultraviolet/near-visible (UV/near-vis)
actinic flux in the snowpack at each location. We have developed a simple
and broadly applicable parameterization to calculate depth and wavelength
dependent snowpack actinic flux that can be easily integrated into large-scale
(e.g., 3-D) models of the atmosphere. The calculated e-folding depths of
actinic flux at 305 nm, the peak wavelength of nitrate photolysis in the
snowpack, are 8–12 cm near the stations and 15–31 cm away (>11 km)
from the stations. We find that the e-folding depth is strongly
dependent on impurity content and wavelength in the UV/near-vis region,
which explains the relatively shallow e-folding depths near stations where
local activities lead to higher snow impurity levels. We calculate the
lifetime of NO<sub>x</sub> in the snowpack interstitial air produced by photolysis
of snowpack nitrate against wind pumping (&tau;<sub>wind pumping</sub>) from
the snowpack, and compare this to the calculated lifetime of NO<sub>x</sub> against
chemical conversion to HNO<sub>3</sub> (&tau;<sub>chemical</sub>) to determine whether
the NO<sub>x</sub> produced at a given depth can escape from the snowpack to the
overlying atmosphere. Comparison of &tau;<sub>wind pumping</sub> and
&tau;<sub>chemical</sub> suggests efficient escape of photoproduced NO<sub>x</sub> in the
snowpack to the overlying atmosphere throughout most of the photochemically
active zone. Calculated vertical actinic flux profiles and observed snowpack
nitrate concentrations are used to estimate the potential flux of NO<sub>x</sub>
from the snowpack. Calculated NO<sub>x</sub> fluxes of
4.4 × 10<sup>8</sup>–3.8 × 10<sup>9</sup> molecules cm<sup>−2</sup> s<sup>−1</sup>
in remote polar locations and 3.2–8.2 × 10<sup>8</sup> molecules cm<sup>−2</sup> s<sup>−1</sup>
near polar stations for January at Dome C and
South Pole and June at Summit suggest that NO<sub>x</sub> flux measurements near
stations may be underestimating the amount of NO<sub>x</sub> emitted from the
clean polar snowpack.